Tomato Hornworms are the biggest caterpillars you ever did see. They'll eat your plants and your mother too. Planting basil near your tomatoes will help discourage hornworms from horning in on your tomato crop. They grow up and become the 5-Spotted Hawk Moth. To do more learnin' about this bug, see wikipedia's article.

Comments:

I knew these things really existed! When I was a kid, my dad grew tomato plants and there were always lots of giant green caterpillars on 'em. I haven't seen one in years, and was starting to credit their existence to an overactive childhood imagination. Yeah—post a photo, Arlen! —ss

Not the best picture — my camera kept wanting to focus on the background instead of the hornworm. This guy was about 2 inches long and about as big around as 2 pencils side by side. I squashed about twenty or thirty of these guys this year. I've never heard of the basil trick, but I had a volunteer basil come up this year right next to my tomatos, and the hornworms are gone. Coincidence? Seasonal? Mystery solved? Hornworms also come in a tobacco variety. —AlexPomeranz

Growing up in North Davis my dad used to (and still does) grow Tomatoes in his seasonal veggie plots, as a kid I would find and remove these worms, they would get pretty damn big —StevenDaubert